Today in an age of energy conservation, low-pressure mercury lamps that exhibit high luminous efficiency and long operating life have received attention. In particular, compact self-ballasted fluorescent lamps have been examined as a light source alternative to the incandescent lamps. Note that, hereafter, compact self-ballasted fluorescent lamps are referred to as ‘lamps.’ On the other hand, fluorescent lamps, such as a compact single-capped fluorescent lamp, are referred to as ‘fluorescent lamps’ in distinction from the compact self-ballasted fluorescent lamps.
Such lamps, each of which comprises an arc tube formed by bending a glass tube, a ballast circuit for lighting the arc tube, and a case housing this ballast circuit therein and having a base. Note here that these are a type of lamp that has no globe encasing the arc tube.
Among arc tubes used for the above lamps, there is a type that is formed with multiple, for example three, U-shaped glass tubes held together. However, there is another type that is increasingly being adopted late years. That is an arc tube formed in the shape of a double spiral. The shape of the double-spiral arc tube can be described as a single glass tube being bent double in approximately the midsection from its both ends and each limb of the bent glass tube being formed into a spiral structure. The reason the double-spiral arc tube is now being adopted is that forming an arc tube into the shape of a double spiral allows for effective use of limited space, and the double-spiral arc tube becomes smaller than an arc tube comprising multiple U-shaped glass tubes. The lamps, which are alternative to a 60 W incandescent lamp, are being reduced in size to almost the same dimensions of the incandescent lamps.
In a lamp utilizing a double-spiral arc tube, the bulb wall loading is set high in order to attain the same luminous flux as a incandescent lamp. For this reason, the cold spot temperature of the glass tube under steady state illumination exceeds the optimum temperature of the glass tube at which the maximum luminous flux is radiated (this optimum temperature is, hereafter, referred to as an ‘optimum cold spot temperature’). As a result, the lamp with a double-spiral arc tube fails to achieve the best luminous efficiency.
In the lamp using a double-spiral arc tube, in order to approximate the cold spot temperature of the glass tube under steady state illumination to its optimum cold spot temperature, for instance, the apical part of the arc tube is bulged so that more area is provided for heat dissipation. Consequently, a 13 W lamp, alternative to a 60 W incandescent lamp (luminous flux: 810 lm), has obtained a luminous flux of approximately 800 lm and a luminous efficiency of 62 lm/W. In the case of a 23 W lamp, alternative to a 100 W incandescent lamp (luminous flux: 1520 lm), a luminous flux of about 1500 lm and a luminous efficiency of 65 lm/W have been attained. Note that both these lamps, a 13 W and a 23 W lamp, have achieved a rated life of 6000 hours or more.
As just described, in the conventional double-spiral arc tube lamps, the cold spot temperature has been lowered by bulging the apical part of the arc tube. However, the cold spot temperature under steady state illumination yet exceeds the optimum cold spot temperature, and therefore the improvement in luminous efficiency is still far from adequate.
Take notice that, for the purpose of lowering the cold spot temperature of the glass tube, if for instance the bulb wall loading is reduced, a desired luminous flux cannot be obtained. Additionally, enlarging the diameter of the glass tube ends up with an increase in size of the arc tube.